Conventionally, in a communication system including a transmitting device, a communication medium, and a receiving device, a physical communication signal transmitting path for transmitting a communication signal and a physical reference point path different from the communication signal transmitting path, the reference point path being to share a reference point for determining a difference in level of the communication signal between the transmitting device and the receiving device, are provided to perform communication (see for example Patent Document 1 or Patent Document 2).
For example, Patent Document 1 and Patent Document 2 describe communication techniques using a human body as a communication medium. In both methods, a human body is used as a first communication path, and direct capacitive coupling between electrodes in a ground or a space is provided as a second communication path, so that an entire communication path formed by the first communication path and the second communication path forms a closed circuit.
[Patent Document 1]    Japanese Patent Laid-Open No. Hei 10-229357
[Patent Document 2]    JP-A-Hei 11-509380